(a) Field of the Invention
The present invention relates to a semiconductor circuit that uses an element having a threshold voltage, and more particularly to a circuit that compensates for the nonuniformity and variations of the threshold voltage.
(b) Description of the Related Art
Semiconductor devices having a threshold voltage, i.e., metal-oxide semiconductor (MOS) transistors, are most widely used. There are various kinds of transistors, for example, a single crystal silicon transistor, a thin film polycrystalline silicon transistor, an amorphous silicon transistor and the like.
Such MOS transistors have a threshold voltage between the gate and source electrodes that turns the device on or off. The threshold voltage is expressed by a thickness of a gate insulation film formed between the gate electrode and a channel area, and the degree of the doping in the channel area, which vary in accordance with the process condition.
The nonuniformity and variations of the threshold voltage may cause a significant error for the circuit operation.
As described above, the threshold voltage is an important factor for an operation of the circuitry. Therefore, an analog circuit employing a polycrystalline silicon transistor or an amorphous silicon transistor having significant nonuniformity or variations in the threshold voltage is hard to be realized.
A method for compensating for a nonuniformity or variations of the threshold voltage outside of the circuit is used when the circuit is made up of single silicon transistors. However, such a method is complicated and may not ensure a precise circuit operation.
FIG. 1 is a circuit view of a conventional common drain MOS transistor.
The conventional MOS transistor includes an n-channel MOS (NMOS) transistor 15 that consists of a gate electrode 12, a drain terminal 11, and a source terminal 13 and has a threshold voltage, and a capacitor 14 connected between the source terminal 13 and an earth terminal. A constant voltage V.sub.DD is supplied to the drain terminal 11.
The conventional transistor operates as follows.
If a voltage higher than a threshold voltage is applied to the gate electrode 12, the transistor 15 is turned on and a current flows from the drain terminal 11 toward the source terminal 13, which charges the capacitor 14 and raises the electric potential of the source terminal 13. When the electric potential reaches the difference between the input voltage of the gate electrode 12 and the threshold voltage of the transistor 15, the transistor 15 is turned off and the difference between the input voltage and the threshold voltage appears in the source terminal 13. In such a structure, if the threshold voltage is not uniform or varies, the output voltage varies corresponding to the variation of the threshold voltage. In other words, the nonuniformity and variations of the threshold voltage cause a distorted output.
FIG. 2 is a circuit view of the conventional transistor consisting of an n-channel MOS and a p-channel MOS.
Such a conventional transistor circuit includes the NMOS transistor 15 consisting of the gate electrode 12, the drain terminal 11, and the source terminal 13 and having a certain threshold voltage, a PMOS transistor 19 consisting of a gate electrode 18, a source terminal 16, a drain terminal 17 and having a certain threshold voltage, a first switching element 1 for switching between the gate electrode 12 of the NMOS transistor 15 and the earth terminal, a second switching element 2 for switching between the input terminal and the gate electrode 12, a third switching element 3 for switching between the source terminal 13 and the output terminal, a fourth switching element 4 for switching between the source terminal 16 and the output terminal, a fifth switching element 5 for switching between the gate electrode 18 and the input terminal, and a sixth switching element 6 for switching between the gate electrode 18 and the constant voltage V.sub.DD applied to the drain terminal 11 of the NMOS transistor 15.
In such a conventional circuitry consisting of the NMOS and PMOS transistors, the output voltage varies corresponding to the degree of the threshold voltage variation if the threshold voltage fluctuates or varies. That is, the threshold voltage fluctuation or variation causes a distorted output.
The conventional MOS transistors have following problems.
MOS transistors have threshold voltages which can vary in accordance with the process condition, substances and thickness of the gate insulation film, and the degree of the doping in the channel area. Therefore, the circuitry using such a MOS transistor may not function properly due to the threshold voltage fluctuation or variations.